As of September 2012, about 85% of American adults own a cell phone, with over half of them owning a smartphone. For the years of 2011 and 2012, the smartphone ownership increased dramatically. One difference between the smartphone and the traditional cell phone is the ability to download and use mobile applications (commonly referred to as apps) that match various interests of individual owners. As a result, the mobile apps market also experienced an explosive growth. The number of apps exceeded one million with significant (e.g., 20% or more) annual increase of apps in major online apps markets. With an average of 50 apps installed on each smartphone and a daily average of 1.4 hours spent on using the apps, the mobile apps and the app markets have become a significant part of people's daily lives.